Journal
JOURNAL OF PHYSICAL CHEMISTRY B
Volume 122, Issue 11, Pages 3008-3014Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcb.8b00740
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Funding
- Defense Threat Reduction Agency [DTRA HDTRA1-15-1-0038]
- Office of Naval Research [ONR N00014-16-1-2681]
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The best-known examples of smart, responsive hydrogels derive from poly(N-isopropylacrylamide) (PNI-PAM) cross-linked polymer networks. These hydrogels undergo volume phase transitions (VPTs) triggered by temperature, chemical, and/or environmental changes. PNI-PAM hydrogels can undergo more than 50-fold volume changes within similar to 1 mu s intervals. Studies have tried to elucidate the molecular mechanism of these extraordinarily large responses. Nevertheless, the molecular reaction coordinates that drive the VPT remain unclear. Using visible nonresonance Raman temperature-jump spectroscopy, we determined the molecular ordering of this VPT. The PNIPAM hydrophobic isopropyl and methylene groups dehydrate with time constants of 109 +/- 64 and 104 +/- 44 ns, initiating the volume collapse of PNIPAM. The subsequent dehydration of the PNIPAM amide groups is significantly slower, as our group previously discovered (360 +/- 85 ns). This determination of the ordering of the molecular reaction coordinate of the PNIPAM VPT enables the development of the next generation of super-responsive materials.
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